Modern gas or combustion turbines must satisfy the highest demands with respect to reliability, weight, power, economy, and operating service life. In the development of such turbines, the material selection, the search for new suitable materials, as well as the search for new production methods, among other things, play an important role in meeting standards and satisfying the demand.
The materials used for gas turbines may include titanium alloys, nickel alloys (also called super alloys) and high strength steels. For aircraft engines, titanium alloys are generally used for compressor parts, nickel alloys are suitable for the hot parts of the aircraft engine, and the high strength steels are used, for example, for compressor housings and turbine housings. The highly loaded or stressed gas turbine components, such as components for a compressor for example, are typically forged parts. Components for a turbine, on the other hand, are typically embodied as investment cast parts.
Although investment casting is not a new process, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment castings more quickly, efficiently, cheaply and of higher quality.
Conventional investment mold compounds that consist of fused silica, cristobalite, gypsum, or the like, that are used in casting jewelry and dental prostheses industries are generally not suitable for casting reactive alloys, such as titanium alloys. One reason is because there is a reaction between mold titanium and the investment mold. Any reaction between the molten alloy and the mold will greatly deteriorate the properties of the final casting. The deterioration can be as simple as poor surface finish due to gas bubbles, or in more serious cases, the chemistry, microstructure, and properties of the casting can be compromised.
There is a need for a simple investment mold that does not react significantly with titanium and titanium aluminide alloys. Approaches have been adopted previously with ceramic shell molds for titanium alloy castings. In the prior examples, in order to reduce the limitations of the conventional investment mold compounds, several additional mold materials have been developed. For example, an investment compound was developed of an oxidation-expansion type in which magnesium oxide or zirconia was used as a main component and metallic zirconium was added to the main constituent to compensate for the shrinkage due to solidification of the cast metal. In addition, in another example, an investment compound in which magnesium oxide and aluminum oxide are used as main components, a fine metallic titanium powder is added in order to reduce the amount of shrinkage of the mold and to compensate for the dimensional error caused by the shrinkage of the cast metal on solidification.
However, the above prior art investment compounds have significant limitations. For example, the investment mold compound that is intended to compensate for the shrinkage due to the solidification of the cast metal by the oxidation-expansion of metallic zirconium is difficult to practice, for several reasons. First, a wax pattern is coated on its surface with the new investment compound with zirconium and then the coated wax pattern is embedded in the conventional investment compound in an attempt to make the required amount of zirconium as small as possible; coating the wax with zirconium is very difficult and not highly repeatable. Second, waxes of complex shaped components can not be coated in a sufficiently uniform manner. In addition, the coated layer can come off the wax when the investment mold mix is placed externally around the coated layer and the pattern, with the result that titanium reacts with the externally placed investment mold mix.
There is thus a need for simple and reliable investment casting methods which allow easy extraction of near-net-shape metal or metal alloys from an investment mold that does not react significantly with the metal or metal alloy.